Using classic Outlook for Windows in business environments
Users that never liked Outlook where using alternative email client even when Outlook 2013 or whatever were a viable option, so?
MS is not forcing anyone to install the new Outlook or buy a subscription, but if you want a free client is there for free.
But if you want to use a MS email account, you need a modern email client that supports OAuth, simple as that.
OAuth has nothing to do with this, you've completely missed the point. The point is that the new Outlook client requires all third party emails to be "cloud cached" onto Microsoft's servers. This data grab is not something Outlook 2013 required, nor indeed any email client Microsoft have ever produced, until now.
Maybe you've only ever used Microsoft accounts with Outlook, in which case this has no effect on you. But for others, sharing all their third-party emails with Microsoft is a pretty big deal.